The development of very high speed integrated circuit (VHSIC) technology has created a need for dielectric materials with low dielectric constant. Dielectric constant (K) is a measure of the insulating ability of a material. It is the ratio of the capacitance of a given material to the capacitance of air for the same thickness. Dielectric constant of air is assigned the value 1. The need for low dielectric constant increases with frequency of the electrical signal. The use of low K dielectric materials minimizes signal loss in the dielectric, crosstalk between signal lines, and signal rise and decay times.
Several approaches to low K materials have received considerable attention, including the use of polymeric materials (K=2-4), glass, glass-ceramics and ceramic materials (K=3.8-6). In addition to dielectric properties, physical, chemical, electrical and mechanical properties must be considered for material selection. Electrical design parameters, packaging, manufacturing and end use requirements further restrict the applicability of many materials.
The lowest dielectric constants can be obtained with polymeric materials. Thermal considerations are the major restriction on utility of polymeric materials. High process and environmental temperatures, thermal expansion and thermal conductivity preclude the use of low K polymeric materials in many applications.
For applications requiring high process or use temperatures, especially 500.degree. C. or above, inorganic materials are required. The lowest K value for such inorganic materials is K=3.8 for SiO.sub.2. Because of the refractory nature of SiO.sub.2, it is usually necessary to use it in combination with other materials, such as glass or glass ceramics, which substantially raises the minimum value of K that can be realized.
For circuitry which is to be printed on 96% Al.sub.2 O.sub.3 substrates, the thermal coefficient of expansion (TCE) of the Al.sub.2 O.sub.3 substrate and the dielectric must be matched to prevent warping and/or cracking of multilayer circuits. In general, for glass and glass ceramic systems, low K usually means low TCE and a high glass softening point. Materials such as cordierite and mullite have a low dielectric constant but are not suitable for use on Al.sub.2 O.sub.3 substrates because of TCE mismatch. Furthermore, compositions containing mullite or cordierite in conjunction with a low softening point glass in general tend to raise TCE, lower the firing temperature, and increase K.
The combination of such material properties with design criteria and end use requirements for a good low K dielectric frequently and severely limit the materials which heretofore could be used. Thus, there is a serious need for low K screen-printable thick film compositions which can be fired conventionally on 96% wt. alumina substrates.